1. Field of the Invention
This invention relates to an acoustic imaging method and apparatus for use in nondestructively inspecting polymer materials in order to determine their reliability and the like.
2. Prior Art Statement
The use of the acoustic imaging method for nondestructive evaluation of materials is not new. However, depending on the material being evaluated and particularly in the case of polymer materials, the attenuation of the high frequency acoustic waves passed through the material is so large as to make use of the acoustic imaging method extremely difficult. Moreover, the attenuation of the acoustic waves increases with increasing frequency. Thus in the inspection of materials exhibiting high attenuation, it has generally been the practice to employ low-frequency acoustic waves. This is disadvantageous, however, since the longer wavelength of low-frequency acoustic waves lowers the resolution with which defects and structural details of the material can be observed.
Nowadays a large amount of bonding is being used in structural materials for aircraft and automobiles, and the presence of bubbles and other defects at the bonded portions can lead to a serious decrease in the material strength. With conventional techniques it has been difficult to detect microdefects down to the smallest size (about 0.1 mm) which can lead to reduced strength. In such technical fields there has therefore arisen an urgent need for an improvement in inspection sensitivity and resolution.
On the other hand, it has been discovered that when a polymer material or a composite material containing a polymer is cooled from room temperature to around -100.degree.) C., it may within specific temperature ranges exhibit a marked decrease the viscoelasticity that is the cause of acoustic waves attenuation. Moreover, since the speed of sound also varies greatly with temperature, in some bonded structures there can be expected to be observed an improvement in acoustic impedance matching.
The present invention was accomplished on the basis of the aforesaid knowledge and one object thereof is to provide an acoustic imaging method for imaging materials which takes advantage of the temperature dependence of the acoustic transmittance of the material for increasing sensitivity and resolution in acoustic imaging. Another object of the invention is to provide an apparatus for carrying out this method.